An image sensor uses an array of pixels to capture an image when the image sensor is exposed to light. FIG. 1 shows a block diagram of an illustrative prior art pixel array 103. Pixel array 103 includes auxiliary circuitry such as drivers, buffers, and multiplexers for the signals in the array. A voltage supply 105 supplies the pixel array 103 with power. At the beginning of an exposure period, a reset signal 107 is asserted to reset some or all of the pixels in the pixel array 103. Consequently, the pixels are charged to a reset voltage, which is typically a function of the voltage supply 105. As the pixel array 103 is exposed to incident light 109, the voltages at each pixel decrease.
At the end of an exposure period, the final voltage of each pixel is compared to its original reset voltage. These voltage swings represent the captured image, and are proportional to the exposure level of the pixel array 103. Large voltage swings indicate a high exposure level, which means that the pixel array 103 was exposed to bright light or had a long exposure period. Conversely, small voltage swings indicate a low exposure level, which means that the pixel array 103 was exposed to dim light or had a short exposure period. The voltage swings are read from the pixel array 103 as image signals 111.
A higher voltage supply increases the dynamic range of a pixel array, because each pixel has a larger reset voltage, and thus a bigger range for the voltage swing. A larger dynamic range allows the pixel array to capture a more faithful image when the exposure level is high. However, both pixel temporal noise and dark current noise (hereinafter, collectively referred to as just “noise” or “pixel noise”) have been found to increase along with the voltage supply when the pixel array is created with complimentary metal oxide silicon (CMOS) technology. The noise distorts the image captured by the pixel array.